Mass, magnetosphere, and life

If one surveys stars, and their planetary systems, of 4.5 - 5 Byrs old; and then detects terrestrial exoplanet of say 1.5 earth masses, then one probably still has a magnetosphere - if we are typical (Copernian Principle). And if in habitat zone, then also look for oxygen signature in atmosphere, from very far distance. So if photosynthesis for such a long time of 5 Byrs (stromatalites 3.5 Byrs ago for us), then would there be some probability of a species developing culture? Then consider size of our galaxy of ~100-200 B stars, and any large number fall outs.

Trying to find a planet that can hold life is one of the reason behind surveys of exoplanets.

If you are asking about the probability of finding one, just in our Galaxy, I would say that chances are quite important. And if we are the "only" ones in our Galaxy, chances are finding some inother galaxies are very close to one.